where do you come from?

Once upon a time (in the 19th century) there was a small band of people living on a small island named Tristan da Cunha that lies in the south Atlantic, roughly between Africa and South America. This group had been a garrison of the British Marines while Napoleon was held in exile on a northerly island, St Helena, just in case he tried to escape. He did indeed escape, but there's not a lot you can do when the ship traveling to communicate this news is a few thousand miles away. Napoleon was long gone by the time the garrison got the message, and so they were no longer needed.

Some of these people decided they really liked this island, and stayed. This consisted of one man, his wife and their daughters, and some other non-relatives. They remained on this island, built families, and their population grew without a whole lot of outside contribution. Here is that island:

It looks quite lovely, but what do you think about the shape of this island? Yes, in 1961, that very volcano blew its top and all the 240 settlers had to be ordered off and rescued by the British Navy, thus, the end of inhabited Tristan da Cunha, for a little while at least. Eventually, some people returned.

Anthropologists have studied populations like this because of what we call the "founder effect". The biology of populations that grew from an isolated group tends to show some very unique genetic features. For this group, one of the few founders had carried a gene for retinitis pigmentosa, causing blindness, which was found 10 times more often compared to that of Britain, from which the original founders had come.

Another location much closer and more familiar to Americans is with Martha's Vineyard in Massachusetts. Here, hereditary deafness was the common condition, and so much so, that the people developed their own sign language, somewhat different from ASL and officially called MVSL. There have been stories about people in the bars, drinking and chatting as most bar-goers do, but as soon as an out-of-town person would come in, they would become silent and switch to their own code of sign language.

Surprisingly, even though these close-knit groups share many common genetic traits, there were often very low rates of inbreeding. Still, what we learn from this is that it's better to mix things up!